Yb3+-codoped oxyhalide tellurite glasses

Yb3+-codoped oxyhalide tellurite glasses

Solid State Communications 133 (2005) 89–92 www.elsevier.com/locate/ssc Upconversion luminescence of Tm3C/Yb3C-codoped oxyhalide tellurite glasses Sh...

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Solid State Communications 133 (2005) 89–92 www.elsevier.com/locate/ssc

Upconversion luminescence of Tm3C/Yb3C-codoped oxyhalide tellurite glasses Shiqing Xua,b,*, Hongtao Suna,b, Shixun Daia, Junjie Zhanga, Zhonghong Jianga a

Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, PR China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China Received 18 August 2004; accepted 10 October 2004 by A. Pinczuk Available online 19 October 2004

Abstract Thermal stability, Raman spectra and blue upconversion luminescence properties of Tm3C/Yb3C-codoped halide modified tellurite glasses have been studied. The results showed that the mixed halide modified tellurite glass (TFCB) has the best thermal stability, the lowest phonon energies and the strongest upconversion emissions. The effect of halide on upconversion intensity is observed and discussed and possible upconversion mechanisms are evaluated. The intense blue upconversion luminescence of Tm3C in TFCB glass may be a potentially useful material for developing upconversion optical devices. q 2004 Elsevier Ltd. All rights reserved. PACS: 78.20.Ke; 85.17.Mi; 42.70.Ce Keywords: A. Oxyhalide tellurite glasses; A. Tm3C/Yb3C-codoped glasses; E. Upconversion luminescence

1. Introduction In recent years, the study of blue lasers for use in color displays, high density optical data storage and reading, biomedical diagnostics, optical communications, infrared lasers viewers and indicators has become a focus of research [1–4]. Tm3C is one of the most studied rare-earth ions for blue laser operation based upon upconversion [5–7]. A number of reports on the upconversion luminescence properties of Tm3C were discussed in a variety of glasses [5–10]. The choice of the host material is very important to obtain high efficient upconversion emission. The glass host with low phonon energy can reduce the nonradiative loss due to multiphonon relaxation and thus yields a strong upconversion signal [11]. Though fluoride glasses have been extensively studied due to their low phonon energies, oxide

* Corresponding author. Tel.: C86 21 59914293; fax: C86 21 59914516. E-mail address: [email protected] (S. Xu). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.10.010

glasses are appropriate for practical applications due to their high chemical durability and thermal stability. Therefore, an objective now in search of upconversion host material is to develop a glass host with low phonon energy and good thermal stability [12]. As is known, glasses based on mix oxide–halide systems combine the good optical properties of halide glasses (a broad range of optical transmittance and low optical losses) with the better chemical and thermal stability of oxide glasses [13,14]. Among oxide glasses, tellurite glasses are investigated extensively due to their relative low-phonon energy, high refractive index, high dielectric constant, good corrosion resistance, thermal and chemical stability and be capable of incorporating large concentrations of rare earth ions into the matrix [15,16]. Therefore, it is expected that halide modified tellurite glasses should bring the interesting properties in the systems. However, there is still no detailed study on blue upconversion luminescence of Tm3C-doped halide modified tellurite glasses. In this letter, we report on the experimental investigation of thermal stability, Raman spectra and blue upconversion luminescence in

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Tm3C/Yb3C-codoped halide modified tellurite glasses. The results demonstrate that they can act as potential materials for developing upconversion optical devices.

2. Experiments The glass samples were prepared according to following composition in mol%: 70TeO2–25ZnO–5Na2O (TZN), 60TeO2–40PbF2 (TF), 60TeO2–20PbF2–20PbCl2 (TFC), 60TeO2–10PbF2–20PbCl2–10PbBr2 (TFCB). All powders are analytical pure except TeO2 (99.99%). Tm2O3 (99.99%) and Yb2O3 (99.99%) doping concentrations are 0.1 and 2 mol%, respectively. About 50 g batches of starting materials were fully and then melted between 700 and 800 8C in covered corundum crucibles in a SiC Globar furnace with an N2 atmosphere. When the melting was completed, the glass liquids were cast into stainless steel plates. The obtained glass sample was cooled to room temperature at a rate of 5 8C/h and then was cut and polished carefully in order to meet the requirements for optical measurements. The glass transition temperature (Tg) and crystallization onset temperature (Tx) were determined by differential scanning calorimetry (DSC) at a heating rate of 10 8C/min, using aluminum oxide ceramic pan. Visible upconversion spectra were obtained with a TRIAX550 spectrofluorimeter pumped by a 980 nm laser diode (LD) with a maximum power of 2 W. In order to compare the luminescence intensity of Tm3C in different samples as accurate as we can, the position and power (100 mW) of the pumping beam and the width (1 mm) of the slit to collect the luminescence signal were fixed under the same condition and the samples was set at the same place in the experimental set-up. The Raman spectra were recorded on a Fourier transform Raman spectrophotometer (Nicolet MODULE). Nd:yttritium– aluminum–garnet operating at 1064 nm was used as the excitation source and the laser power level was 500 mW. In order to compare the stretching vibrations of the different composition samples as accurate as we can, the laser power level and scan frequency and definition were fixed under the same condition. All the measurements were taken at room temperature.

Fig. 1. Tx, Tg and DTZTxKTg of TZN, TF, TFC and TFCB glasses.

Fig. 2 presents the Raman spectrum for undoped TZN, TF, TFC and TFCB glasses. For TZN glass, the bands around 673 and 756 cmK1 are assigned to stretching vibrations of TeO4 and TeO3 (and/or TeO3C1) groups, respectively [19,20]. The band around 443 cmK1 is assigned to bending vibrations of Te–O–Te linkages [21]. For halide modified tellurite glasses, important changes are observed in the spectra compared with TZN glass. First, the maximum phonon energy in TZN glass is 756 cmK1, while those in TF, TFC and TFCB glasses are 742, 738 and 732 cmK1, respectively. Secondly, the band intensities in the intermediate region of the spectra from 250 to 550 cmK1 in TF, TFC and TFCB glasses are lower than that in TZN glasses and the new bands are observed in halide modified tellurite glasses due to addition of PbF2, PbCl2 and PbBr2. Finally, another important change is in low frequency region (below 200 cmK1) of the spectra, where the intensity for TF, TFC and TFCB glasses are much stronger than those in TZN glasses. From the above results we can deduce that halide ions have an important influence on formation of glass network. Layne et al. [22] have discussed the multiphonon

3. Results and discussion Fig. 1 shows Tx, Tg and DTZTxKTg of TZN, TF, TFC and TFCB glasses. The quantity, DTZTxKTg, has been frequently used as a rough estimate of the glass stability. To achieve a large working range of temperature during our sample fiber drawing, it is desirable for a glass host to have DT as large as possible [17,18]. It can be seen that the DT value of TFCB glass is larger than those of TF, TFC, TZN glasses, indicating the mixed halide modified tellurite glasses with good thermal stability.

Fig. 2. The Raman spectrum for undoped TZN, TF, TFC and TFCB glass.

S. Xu et al. / Solid State Communications 133 (2005) 89–92

relaxation of rare-earth ions in oxide glasses. The approximate frequencies of the high-energy phonons in each glass are: borate: 1350 cmK1, phosphate: 1300 cmK1, silicate: 1100 cmK1, germanate: 900 cmK1 and tellurite: 800 cmK1 [23]. Therefore, the multiphonon rate of Tm3C in halide modified tellurite glasses can be lowest in TFCB glass. Therefore, the upconversion luminescence rate of Tm3C in TFCB glass could be highest in the above glass systems. The room temperature upconversion luminescence spectra of Tm3C/Yb3C-codoped TZN, TF, TFC and TFCB glasses in the wavelength region from 400 to 700 nm are shown in Fig. 3. Two emission bands centered around 475 and 649 nm corresponding to the transitions 1 G4/3H6 and 1G4/3H4 of Tm3C, respectively, were simultaneously observed. Clearly, the red emission around 649 nm is relatively weaker than the blue emission around 475 in Tm3C/Yb3C-codoped TZN, TF, TFC and TFCB glass samples. For the TZN, TF, TFC and TFCB glasses, the upconversion intensities of the blue emission around 475 nm are 0.68, 2.73, 3.53, 3.81, respectively, while those of the red emission around 649 nm are 0.08, 0.28, 0.41, 0.53, respectively. The upconversion intensity of blue (475 nm) and red (649 nm) emissions in TFCB glass increases by a factor of about 5.6 and 6.6, respectively, when compared with those in TZN glass. The fact confirms the expectation that halide ions with low phonon energy contribute much to the increase of upconversion luminescence intensity at room temperature. The results showed that the dominant emission in TFCB glass is the blue emission. The blue emission in the region 450–510 nm accounts for about 90% of the total emitted light in the spectral region studied. It is important to mention at this point that the blue emission around 475 nm in TFCB glass was intense enough to be seen the naked eye at excitation power as low as 40 mW. According to the energy matching conditions, the

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possible upconversion mechanisms for the blue and red emissions are discussed based on the energy level of Tm3C and Yb3C presented in Fig. 4 [24,25]. The excitation process for the 1G4/3H6 and 1G4/3H4 transitions can be explained as follows. In the first step, a 980 nm photon is absorbed by Yb3C, which provokes the 2F7/2/2F5/2 transition and then the excitation of Tm3C in the 3H5 is involved by means of the energy transfer (ET) mechanism of excited Yb 3C to Tm 3C:ET:2F5/2(Yb 3C )C 3H 6 (Tm3C)/2F7/2(Yb3C)C3H5(Tm3C). In the second step, Tm3C in the 3H5 excited state relaxes nonradiatively to the metastate level 3H4. Tm3C in the 3H4 level is excited to 3F2,3 level by ET from Yb3C and absorption a photon. Thus, the population of 3F2,3 is based on the processes as follows: ET from Yb3C:2F5/2(Yb3C)C3H4(Tm3C)/2F7/2 (Yb 3C )C 3F2,3(Tm3C ) and excited state absorption: 3 H4(Tm3C)Ca photon/3F2,3(Tm3C). Then the 3F2,3 states also relaxes by a multiphonon assisted process to the 3F4 level. Finally, Tm3C in the 3F4 level is excited to 1G4 level by ET from Yb3C and absorption a photon. Therefore, the population of 1G 4 is based on the processes as follows: ET from Yb3C:2F5/2(Yb3C)C3F4(Tm3C)/2F7/2 (Yb 3C )C 1G 4(Tm 3C) and excited state absorption: 3 F4(Tm3C)Ca photon/1G4(Tm3C). From the 1G4 level, the Tm3C ions decay radiatively to the 3H6 ground state generating the intense blue emission around 475 nm. The major contribution to the red (649 nm) emission is attributed to the 1G4/3H4 transition, while the transition probability involved in the above processes is small, the red emission observed is weak. From the above results it can be concluded that a three-phonon upconversion process is responsible for blue (475 nm) and red (649 nm) emission.

4. Conclusion We have studied thermal stability, Raman spectrum and

Fig. 3. Upconversion luminescence spectra of Tm3C/Yb3Ccodoped TZN, TF, TFC and TFCB glasses under 980 nm excitation at room temperature.

Fig. 4. Simplified energy level diagram of Tm3C and Yb3C and possible transition pathways yielded under 980 nm excitation. The dashed arrows represent the energy transfer processes. The wave lines stand for multiphonon nonradiatiove relaxation processes.

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blue upconversion luminescence properties of Tm3C/Yb3Ccodoped halide modified tellurite glasses. The results showed that TFCB glass have the best thermal stability, the lowest phonon energy and the strongest upconversion luminescence intensity, which confirms that mixed halide has an important influence on formation of glass network. The upconversion mechanism analyses showed that a threephonon upconversion process is responsible for blue (475 nm) and red (649 nm) emission. The data present in this study might provide useful information for further development of Tm3C/Yb3C-codoped glass hosts for upconversion lasers.

Acknowledgements This work was financially supported by Shanghai Science and Technology Foundation (022261046), Chinese National Natural Science Foundation (60207006) and Qimingxing project of Shanghai Municipal Science and Technology commission.

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